Network Entry Procedure During Initialization of Mobile Station in Wireless Access System
FIG. 1 is a flow chart illustrating a network entry procedure when a user equipment (UE) is initialized in a broadband wireless access system.
(1) When initially powered on, a UE searches for a downlink channel and acquires uplink/downlink synchronization with a base station (BS). In this case, the UE acquires uplink/downlink channel parameters by receiving a downlink-MAP (DL-MAP) message, an uplink-MAP (UL-MAP) message, a downlink channel descriptor (DCD) message, and an uplink channel descriptor (UCD) message.
(2) The UE performs ranging with the BS to adjust uplink transmission parameters and receives a basic management connection identifier (CID) and a primary management CID from the BS.
(3) The UE negotiates basic capabilities with the BS.
(4) The UE implements authorization.
(5) The UE registers itself with the BS. The UE managed by an Internet Protocol (IP) receives a secondary management CID from the BS
(6) The UE establishes IP connectivity.
(7) A current date and time are established.
(8) Constituent files of the UE are downloaded from a trivial file transfer protocol (TFTP) server.
(9) Connection for a prepared service is established.
A physical layer of a broadband wireless access system is divided into a single-carrier type and a multi-carrier type. The multi-carrier type uses orthogonal frequency division multiplexing (OFDM), and introduces orthogonal frequency division multiple access (OFDMA) as an access method capable of allocating resources in units of subchannels grouping a part of carriers.
In an OFDMA physical layer, active carriers are separated into groups and the separated carriers are transmitted to different receiving ends. A group of carriers transmitted to one receiving end is called a subchannel. Carriers constituting each subchannel may be adjacent to each other or may be separated from each other at regular intervals. Since such multiple accesses in units of subchannels are possible, a frequency diversity gain, a gain caused by concentration of power, and forward power control can be efficiently performed although complexity of implementation is increased.
A slot allocated to each user is defined by a data region of a two-dimensional time-frequency space and represents a set of successive subchannels allocated by bursts. In OFDMA, one data region is illustrated by a rectangular determined by time coordinates and subchannel coordinates. Such a data region is allocated to uplink of a specific user or, in downlink, can be transmitted by a BS to a specific user. To define the data region in two-dimensional space, the number of OFDM symbols in a time domain and the number of successive subchannels starting at a position separated from a reference point by offset should be determined.
Frame Structure of OFDMA Physical Layer of Broadband Wireless Access System
FIG. 2 illustrates a frame structure of an OFDMA physical layer of a broadband wireless access system. A downlink sub-frame is started with a preamble used for synchronization and equalization in a physical layer A structure of the whole frame is defined through a DL-MAP message and a UL-MAP message of a broadcast form defining the locations and usage of bursts allocated to downlink and uplink.
The DL-MAP message defines usage allocated to each burst with respect to a downlink interval in a burst mode physical layer. The UL-MAP message defines usage of bursts allocated with respect to an uplink interval. Information elements constituting the DL-MAP message determine downlink traffic intervals of a user stage by a downlink interval usage code (DIUC), a CID, and location information of a burst including subchannel offset, symbol offset, the number of subchannels, and the number of symbols. Information elements constituting the UL-MAP message determine usage by an uplink interval usage code (UIUC) according to a CID and define a location of a corresponding interval by a ‘duration’. Usage of each interval is determined according to a UIUC value used in the UL-MAP message and each interval is started with a point separated by ‘duration’ defined in the information elements of the UL-MAP message from a start point of previous information elements.
Ranging
In a process of performing the initial network registration procedure as illustrated in FIG. 1, a process of UE adjusting transmission parameters (frequency offset, time offset, transmission power, etc.) for uplink communication with the BS is called initial ranging. After the network registration procedure completed, the UE performs periodic ranging to continue to maintain uplink communication with the BS. In addition, ranging includes handover ranging for simplifying a procedure when the UE performs a handover operation and bandwidth request ranging for requesting an uplink bandwidth when data to be transmitted by the UE is generated.
In a broadband wireless access system, a code division multiple access (CDMA) code set used for ranging and a region for transmitting CDMA codes are allocated through UL-MAP by a network according to each ranging type. For handover ranging for example, a UE should request handover ranging by selecting a specific code among CDMA codes for the handover ranging and by transmitting the selected code to a network through initial ranging and handover ranging regions. By this method, the network can discriminate between ranging types through the received CDMA code and a CDMA code transmission region.
In IEEE 802.16 (refer to IEEE P802.16Rev2/ID2, “DRAFT Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Broadband Wireless Access Systems”, December, 2007), a ranging structure includes an initial/handover (HO) ranging and periodic/bandwidth request (BR) ranging, as illustrated in FIGS. 3a to 3d. To establish initial uplink time synchronization, a UE uses initial ranging. Handover ranging is used for handover. Time and frequency synchronization is updated through periodic ranging, and resources are requested through bandwidth request ranging. These four types of ranging have different codes. A ranging code is generated using a pseudorandom-noise (PN) code generation equation of 1+X1+X4+X7+X15. A seed of such a pseudo random binary sequence (PRBS) uses {b14 . . . b0=0, 0, 1, 0, 1, 0, 1, 1, s0, s1, s2, s3, s4, s5, s6}, where s6 denotes the least significant bit (LSB) of the PRBS seed, and s6:s0=UL_PermBase (where s6 is the most significant bit (MSB) of UL_PermBase. A total of 256 codes can be generated using the PN generation equation and theses codes are divided according to usage. The first N codes are used for initial ranging, the following M codes are used for periodic ranging, the next L codes are used for bandwidth request ranging, and the following O codes are used for handover ranging.